1. Field of the Invention
The present invention relates to a dielectric lamination structure (dielectric stacked structure) and a wiring board having a dielectric lamination structure. More specifically, the invention relates to a capacitor incorporated in a wiring board as well as to such a wiring board.
2. Description of the Related Art
In recent years, with the requirements of increase in functionality and reduction in size and weight on electronic equipment, the integration density of electronic components such as ICs (integrated circuits) and LSIs (large scale integration) have increased rapidly and those electronic components have come to operate at increasingly high speeds. With this trend, wiring boards which are mounted with electronic components are required to be even higher in wiring density and have even more terminals than before.
Incidentally, in an IC which operates at high speed, when many devices are switched simultaneously at high speed, an inductance component between a power source and the IC obstructs high-speed operation of the IC because all of necessary high-frequency currents are supplied from the power source. As one countermeasure against this problem, that is, to supply the IC with charge necessary for its operation, a capacitor is provided in a wiring board, whereby a local power source is provided in the vicinity of the IC. Charge is stored in the capacitor in a DC sense and the capacitor stably supplies the IC with charge necessary for its operation.
A technique that a capacitor is incorporated in a wiring board (i.e., “in the vicinity of” an IC) has been proposed. In this technique, the capacitor is disposed inside an insulating resin layer of build-up layers which are formed on a board core because disposing the capacitor closer to the IC can make the wiring resistance and the inductance component lower.
The insulating resin layer of the build-up layers is thin, it would be natural that the thickness of the capacitor which is disposed inside the insulating resin layer be made small. However, if the capacitor were unduly thin, it would become insufficient in rigidity to cause a warp, cracks, or the like. For example, this raises a problem that the process of incorporating the capacitor into the wiring board becomes difficult to execute. For example, JP-A-2004-134806 (corresponding to EP 1 408 520 A2) discloses a structure which is produced by forming a first thick-film dielectric having through-holes on a metal foil, then forming a first electrode having through-holes on the dielectric by screen printing, and finally firing the metal foil, the first thick-film dielectric, and the first electrode at the same time. This structure can prevent development of cracks and separation of the dielectric and the first electrode due to the difference in thermal expansion coefficient when the dielectric and the first electrode are fired at the same time.
3. Problems to be Solved by the Invention
However, the technique of JP-A-2004-134806 (corresponding to EP 1 408 520 A2), which uses the thick-film dielectric, cannot prevent development of cracks and separation which are caused by a warp or the like of the capacitor itself which occurs due to, for example, the difference between the thermal expansion coefficients of the capacitor and the insulating resin layer of the build-up layers in the case where a thin dielectric is formed so that the capacitor is incorporated in a wiring board. Furthermore, the use of the thick-film dielectric is contrary to the requirement of reduction in size and weight. If the capacitor has an entirely flat structure, peeling may occur at the boundary between the insulating resin material and the capacitor when the capacitor is incorporated into (mounted on) the insulating resin layer of the build-up layers. In addition, there may occur an event that air bubbles enter the boundary between the capacitor and the insulating resin material at the time of mounting.
Where the first electrode is formed on the dielectric by screen printing, there may occur an event that a printing mask for formation of the first electrode is deviated from the dielectric and, resultingly, conductor paste to form the first electrode goes into the through-holes of the dielectric to render the metal foil and the first electrode electrically continuous with each other. This raises a problem that such an event lowers the production yield and makes it impossible to secure sufficient reliability of the capacitor.